1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method. More specifically it relates to an odd-numbered pass bidirectional printing method employed in serial type printing apparatus.
2. Description of the Related Art
In recent years, relatively inexpensive office automation devices such as personal computers and word processors have come into widespread use. At the same time, efforts are being made to develop various types of printing apparatus that output information supplied from these devices and to enhance printing speed and print quality of these printing apparatus. Among others, a serial type ink jet printing apparatus is drawing attention as a relatively small printing apparatus capable of producing prints at low cost at high speed or with high quality. Such a serial type ink jet printing apparatus can perform a bidirectional printing to produce an image at high speed or perform a multi-pass printing to produce an image with high quality. Brief descriptions are made in the following as to the bidirectional printing and multi-pass printing in the serial type ink jet printing apparatus.
(Bidirectional Printing)
In a serial type ink jet printing apparatus, a print head having an array of ink drop ejection nozzles integrally formed therein is mounted in a carriage that is moved in a main scan direction in the printing apparatus. Individual nozzles (or ejection ports) of the print head eject ink according to image data as the carriage is moved, to form one band of image. A printing main scan (also referred to simply as a printing scan) of one band and an operation to convey a print medium one band width are alternated repetitively to form one band of image after another on the print medium.
The bidirectional printing is a printing method that, after completing a forward printing scan and the subsequent print medium convey operation, performs a printing scan in the backward direction. Compared with a one-way printing that repeats the process of performing the backward scan without printing operation followed by the printing scan, the bidirectional printing can shorten the printing time. For example, suppose an entire area of the A4-size print medium is to be printed using a print head that has 64 nozzles arrayed therein at a density of 360 dpi (dots/inch) in the print medium conveying direction. In that case, while the one-way printing requires about 60 reciprocal scans including backward scans without printing operation, the bidirectional printing needs only about 30 such reciprocal scans to complete the printing. This means the bidirectional printing can produce an image at almost twice the speed of the one-way printing.
(Multi-Pass Printing)
In a printing operation using a print head having a plurality of nozzles, the quality of an image produced is affected by ejection characteristics of individual nozzles. In a process of manufacturing nozzles of the print head, there inevitably occur some variations in the heating characteristics of electrothermal transducers (heaters) installed in the nozzles that generate an ejection energy and also in the shape of ejection openings. These variations influence the ejection volume and direction of ink ejected from the nozzles, which in turn generates density unevenness and stripes in an image formed on a print medium.
FIGS. 1A-1C show a printing state of a print head that has no ejection characteristic variations. In the figures, reference number 201 represents a print head which, for the sake of simplicity, is shown to have only eight nozzles 202 here. As shown in FIG. 1A, if the sizes and the ejection directions of ink droplets 203 ejected from nozzles are aligned, an arrangement of dots formed on a print medium are to be such as shown in FIG. 1B and a density unevenness in the direction of nozzle array are uniform as shown in FIG. 1C respectively.
FIGS. 2A-2C show a printing state of a print head that has ejection characteristic variations. The sizes and ejection directions of ink droplets ejected from individual nozzles 202 vary as shown in FIG. 2A. The dot arrangement on a print medium is also not uniform, as indicated in FIG. 2B. It is seen that there are some areas where dots overlap each other more than necessary and also blank areas where an area factor is less than 100%. As a result, the density unevenness in the direction of nozzle array is uneven, as shown in FIG. 2C. These non-uniform areas, if repeated in the sub-scan direction, are recognized as density unevenness.
FIGS. 3A-3C show a printing state when a multi-pass printing is done using the print head of FIGS. 2A-2C. As shown in FIG. 3A, the multi-pass printing completes a printing operation on an area that in a one-pass printing can be printed in a single printing scan, by dividing the printing scan into a plurality of printing scans. Here is shown a 2-pass printing method.
FIGS. 4A-4C show an arrangement of dots permitted to be printed by the individual nozzles in three consecutive printing scans. FIG. 4A shows dots permitted to be printed in the first printing scan. Here is shown about half the number of dots printed in this area of print medium and they are arranged on alternate pixels in vertical and horizontal directions. After the first printing scan, the print medium is conveyed half the printing width of the print head (equivalent to 4 dots in this case) in the sub-scan direction. In the subsequent second printing scan the remaining half of the dots that are also arranged on alternate pixels are printed (FIG. 4B) It is noted that they are printed at positions complementary to those dots printed in the first printing scan, i.e., they are printed where dots were not printed in the first printing scan. After another 4-dot conveying operation is finished, about half the dots are again printed in the third printing scan at positions complementary to those dots printed in the second printing scan (FIG. 4C). By repeating the above printing scan and the conveying operation alternately, an image is formed on the same image area (each unit image area) on a print medium by two printing scans of different parts of the print head.
The multi-pass printing described above prevents the dots printed by one nozzle from being connected in line in the main scan direction as shown in FIG. 2B. That is, the multi-pass printing allows the use of a print head equivalent to the print head 201 of FIG. 2A and can still halve adverse effects the ejection characteristic variations among the nozzles have on the print medium image, with a resultant dot arrangement being as shown in FIG. 3B. As a result, the density unevenness in the nozzle alignment direction is almost uniform as shown in FIG. 3C.
FIG. 23 is a schematic diagram for explaining a mask pattern capable of using for 2-pass printing described FIG. 4A to 4C and a completing relationship of the mask. P0001 denotes nozzle array consist of 8 nozzles for ejecting ink of same color. The nozzle array is divided into a first block and a second block each including 4 nozzles. P0002A and P0002B denote mask patterns corresponding to the first block and the second block respectively and each mask pattern has 4 pixels×4 pixels area. P0002A (lower pattern in FIG. 7) is a mask pattern used for a first scan, and P0002B (upper pattern in FIG. 7) is a mask pattern used for a second scan. Each mask pattern (P0002A and P0002B) consist of arrangement of print permitted pixels indicated by black and print non-permitted pixels indicated by white. The mask pattern P0002A for the first scan and the mask pattern P0002B for the second scan have completing relationship each other. Therefore, superimposing them, all of 4 pixels×4 pixels area is filled, and up to 100% printing become possible. Then, as such mask pattern is used repeatedly for the main scan direction 2-pass printing becomes possible for all of area where the print head scans.
Next, the “print permitted pixel” and the “print non-permitted pixel” will be described. The “print permitted pixel” means a pixel in which a dot is permitted to be printed. That is, when a 2-value image data corresponding to the “print permitted pixel” indicates ejecting ink, a dot is printed to the pixel. And when the 2-value image data indicates not-ejecting ink, a dot is not printed to the pixel. On the other hand, the “print non-permitted pixel” means a pixel in which a dot is not permitted to be printed regardless of the 2-value image data. That is, even if the 2-value image data corresponding to the “print non-permitted pixel” indicates ejecting ink, a dot is not printed to the pixel.
P0003 and P0004 denote an arrangement of dots in an image which is completed by 2-pass printing. In the first scan, 2-valued image data generated by using mask pattern P0002A is printed by the first block. Then, the print medium is conveyed, in the direction of an arrow, by a distance corresponding to width of one block. In the following second scan, in a similar way, 2-valued image data generated by using mask pattern P0002A is printed by the first block. At the same time, in the second scan, 2-valued image data generated by using mask pattern P0002B is printed by the second block. In this way, a printing for an area corresponding to half of nozzle arraying region capable of being used in a 2-pass printing mode, is completed by 2 times printing scans.
Although in the above explanation dots have been described to be arranged at alternate pixels in both vertical and horizontal directions in each printing scan, the multi-pass printing is not limited to such a dot arrangement. The positions at which dots are printed in each printing scan are generally determined by an arrangement of print permitted pixels in a mask pattern. It is therefore possible to adjust the dot arrangement and the print permitted ratio by changing the arrangement and ratio of print permitted pixel in the mask pattern. It is noted that, the “print permitted ratio” determined by a mask pattern is a ratio, which is expressed in percentage, of a number of print permitted pixels of a total number of the print permitted pixels and print non-permitted pixels in the mask pattern.
The 2-pass printing has been described in the above. The multi-pass printing may increase the number of passes to 3, 4 and 5 passes to enhance the uniformity of image quality. An increase in the number of passes, however, results in a reduction in the printing speed. So, many printing apparatus has a plurality of print modes with different number of passes, such as one that gives priority to image quality and one that places importance on printing speed. By using the bidirectional printing described earlier, it is possible to strike a balance between the image quality and the printing speed to provide a more appropriate print mode. It should, however, be noted that when a bidirectional multi-pass printing is performed using an odd number of passes in, a new problem that does not emerge in a multi-pass printing with an even number of passes arises.
FIGS. 5A and 5B are schematic diagrams showing a difference between an even-numbered-pass printing (with 4 passes) and an odd-numbered-pass printing (with 3 passes).
The bidirectional printing performs a printing operation in both the forward scan and backward scan. If the print heads for a plurality of inks are parallelly arranged in the main scan direction, the order in which the inks are applied to a print medium during the backward scan is reverse to that of the forward scan. For example, if during a forward scan inks are applied in the order of black, cyan, magenta and yellow, the backward scan applies inks in the order of yellow, magenta, cyan and black. At this time, even if the plurality of ink colors are ejected in the same percentages in both the opposite scans to produce the same image colors, there inevitably occurs some color difference between an image obtained in the forward scan and an image obtained in the backward scan. Further, if the printing is done using a single color or the print heads for a plurality of ink colors are arranged in the sub-scan direction, some printing characteristic differences, such as differences in dot shape resulting from satellite landing position variations, emerge between the forward scan and the backward scan. As a result, there is some density differences between images formed in the forward scan and the backward scan.
Thus, even where the multi-pass printing is performed, it is desired that there be no difference in the number of dots between the forward scan and the backward scan. Take FIG. 5A for example; in the case of an even-numbered-pass printing with four passes, the forward and backward scans are executed two times each over the same image area of a print medium: the same image area being a unit area having a width corresponding to a conveying distance of the print medium between pass and pass. Therefore, if the each printing scans for the same image area is given a print permitted ratio of 25%, the total print permitted percentage of the forward scans and that of the backward scans are both 50%.
However, in the case of an odd-numbered-pass printing with three passes shown in FIG. 5B, the numbers of times that the forward scan and the backward scan are executed over the same image area (unit area) of a print medium are not equal. The same image areas (unit areas) printed by two forward scans and one backward scan and the same image areas (unit areas) printed by one forward scan and two backward scans are alternated in the sub-scan direction. That is, if the print permitted ratio for each printing scan is uniformly set at 33.3%, then image areas with a strong printing characteristic of forward scan where the number of dots printed by the forward scan is 33.3% more than that of the backward scan and image areas with a strong backward scan printing characteristic where the number of dots printed by the backward scan is 33.3% more than that of the forward scan, are formed alternately. Since colors and densities may differ between these two kinds of image areas, overall image impairments such as color unevenness and density variations are likely to occur.
The image impairments described above caused by the bidirectional printing with an odd number of passes emerge with an increasing distinctiveness as the number of passes decreases. That is, a three-pass bidirectional printing with a print permitted ratio difference of 33.3% between the sum of forward scans and the sum of backward scans makes the image impairments most noticeable. If the print permitted ratio in each printing scan is equally set, the print percentage difference decreases to 20% and 14.3% as the number of passes increases to 5 passes and 7 passes, making the image impairments less noticeable.
As to the bidirectional printing with an odd number of passes, Japanese Patent Laid-Open No. 2000-108322 discloses a construction in which a print permitted ratio is differentiated according to nozzle positions in the print head in order to make the sum of print permitted ratios in forward scans and the sum of print permitted ratios in backward scans equal.
FIG. 6 is a schematic diagram showing print permitted ratios of forward scans and backward scans in 3-pass bidirectional printing disclosed in Japanese Patent Laid-Open No. 2000-108322. According to this patent document, a nozzle array of the print head is divided into three blocks, with both side blocks assigned a print permitted ratio of 25% each and a central block assigned a print permitted ratio of 50%. With this arrangement, areas printed by forward scan followed by backward scan followed by forward scan and areas printed by backward scan followed by forward scan followed by backward scan can both have equal numbers of dots capable of being printed by the forward scans and the backward scans. If these numbers of dots cannot be made perfectly equal as shown in FIG. 6, the print permitted ratios of the three divided blocks of the print head nozzle array can be determined in a way that suppresses a difference between the number of dots printed by the forward scan and the number of dots printed by the backward scan.
FIG. 7 is a schematic diagram showing a nozzle array of the print head divided into three blocks, of which upper and bottom blocks are given a print permitted ratios of 30% and a central part 40%. This arrangement can suppress the difference in print permitted ratio between the forward scans and the backward scans to about 20%, if not 0%. If a mask used has too large difference in a print permitted ratio between the central block and end block of the nozzle array, the intended effect of the multi-pass printing of “making the ejection characteristics of individual nozzles less noticeable on a printed image” is lost. This also gives rise to a possibility that the print head longevity may be shortened to a level similar to the life of a nozzle with a large ejection frequency. Thus, it is preferred to use a mask, such as described earlier, that makes inconspicuous image impairments caused by differences in print permitted ratio between forward scans and backward scans and which keeps the print permitted ratio differences small. That is, for providing benefits of multi-pass printing or head longevity described above, the mask pattern of FIG. 7 with small difference in print permitted ratios between forward scans and backward scans has more effective than the mask pattern of FIG. 6 with large difference in print permitted ratios between forward scan and backward scan.
In the following, a mask pattern in which a print permitted ratio of at least one printing scan of plural scans is different from that of other scans, as described above, is referred to as a stepping mask. That is, the stepping mask is a mask wherein print permitted ratios of each printing scans are not equal. On the other hand, a conventional commonly used mask that sets print permitted ratios of different printing scans equal is referred to as a flat mask.
In an ink jet printing apparatus that ejects ink from the print head to print an image, ink droplets ejected from the nozzles are not always stable as they leave the nozzles. When ink is ejected as a droplet from a nozzle opening, a main droplet of a relatively large volume, which is ejected first, is often followed by a smaller, slower sub droplet. Since the print head performs ejection as it moves relative to the print medium, the sub droplets which are slower than the main droplets land on the print medium at positions deviated from the main droplets in the direction of movement of the main scan, forming small dots—satellites.
FIG. 8 is a schematic diagram showing a positional relation on a print medium between a main dot formed of a main droplet and a satellite formed of a sub droplet. The diagram shows that the satellite position with respect to the main dot position during the backward scan is reverse to that of the forward scan. That is, when a bidirectional multi-pass printing is executed, dots printed by the forward scan and dots printed by the backward scan mix together in the same image area (e.g., in the same pixel, on the same pixel line or in the same M×N pixel area).
Such a satellite, if it occurs, will get printed at the same position as the main dot or, if it is small enough compared with the main dot, will not pose any problem to the image quality. However, in the case of print heads that eject high-resolution, small droplets of ink, such as those developed in recent years, main dots themselves are small in diameter, making the presence of satellites not negligible. When two kinds of ink are overlapped to produce a secondary color, in particular, the problem becomes worse.
FIGS. 9A and 9B show how cyan and magenta dots are overlapped to produce a blue color. FIG. 9A shows a printing state wherein two blue dots are formed in a 2×2 pixel area by moving a carriage in a forward direction of arrow. FIG. 9B shows a printing state wherein two blue dots are formed in a 2×2 pixel area by moving the carriage in a backward direction of arrow. Here, it is assumed that two print heads of cyan and magenta have the same satellite generation conditions. By the side of the blue dots (second color main dots) formed of main droplets, satellites (second color satellites) are shown to be formed by two overlapping color dots. These second color satellites formed of two overlapping color dots are more conspicuous than first color satellites and therefore more likely to affect the image quality. Additionally, in each pixel in FIGS. 9A and 9B, the second color satellites placed in one side of the main dots, so the satellites are distributed unevenly. Unevenly distributed, conspicuous satellites inevitably make the printed image look more granular and lose uniformity, degrading the image quality.
A technique to overcome the uneven distribution is disclosed in Japanese Patent Laid-Open No. 2007-38671. Japanese Patent Laid-Open No. 2007-38671 discloses a construction in which satellites of two types of the inks (cyan and magenta) in the same pixel are printed at symmetric positions with respect to main dots.
However, concrete configuration of preferred mask pattern capable of being used for odd-numbered-pass bidirectional printing is not mentioned in Japanese Patent Laid-Open No. 2007-38671. In this way, regarding the conventional mask pattern for odd-numbered-pass printing, positions of satellites of a plurality of dots printed at the same position are not to be considered.
FIG. 10 is a schematic diagram showing an example case in which a same stepping mask is used for both cyan and magenta. Considering an image area, 30% printing is performed in a first scan for both cyan and magenta ink. In a second scan whose direction is opposite that of the first scan, 40% printing is executed. In a third printing scan whose direction is the same as that of the first scan, 30% printing is performed. Since the same mask pattern is used for cyan and magenta, cyan dot and magenta dot landing a same pixel are printed by scans in a same direction. In this case, as a blue image is constructed such as showed in FIG. 9A or FIG. 9B, the satellites are distributed unevenly causing an image degradation.
In this way, in the conventional technologies, an odd-numbered-pass bidirectional printing fails to distribute satellite landing positions uniformly. As a result, with the odd-numbered-pass bidirectional printing, it was impossible to solve the problem of image impairments caused by biased position of satellites. Additionally, it was also impossible to solve both the problem of image impairments caused by a difference in print permitted ratio between forward scans and backward scans and the problem of image impairments caused by a biased position of satellites.